It has long been recognized that in a proposed method for production of chlorine by reacting sulfur trioxide with salt (sodium chloride) a possible intermediate product is sodium chloro-sulfonate. Some prior processes claim to have avoided the production of the intermediate product by conducting the reaction at a sufficiently high temperature so that the sodium sulfate, chlorine and sulfur dioxide are produced directly.
H. F. Johnstone in his article "Chlorine Production -- Nonelectrolytic Processes" in the September 1948 issue of Chemical Engineering Progress (page 657, et seq.) reviews this process using sodium chloride as the metallic halide and points out that at room temperature the ratio of sulfur trioxide to sodium chloride is greater than 2.0 and below 110.degree. C. a definite compound NaCl(SO.sub.3).sub.2 may be formed. He further states that the mono chloro-sulfonate is stable between 110.degree. C. and 225.degree. C. above which temperature range it begins to decompose to pyrosulfate and chlorine with the evolution of chlorine and sulfur dioxide. He further points out that the initial reaction of the sulfur trioxide with salt is exothermic and that the decomposition reaction of the sodium chloro-sulfonate is endothermic. Johnstone suggests that the production of the sodium chloro-sulfonate requires jacketed and cooled reactors to maintain the temperatures below the decomposition temperatures since a large amount of heat is liberated in the reaction. With respect to decomposition he mentions that others have suggested that because chloro-sulfonate has a tendency to foam and become pasty the use of salt cake reactors may become impossible. Further, he discusses and explains the difficulties in the separation of the sulfur dioxide and chlorine produced by the process.
N. A. Laury in his U.S. Pat. No. 2,254,014 suggests that the decomposition of sodium chloro-sulfonate at temperatures between 150-350.degree. C. forms sodium pyrosulphate (which can be neutralized with sodium carbonate) and thionyl chloride which reacts with air to form sulfur dioxide and chlorine. Laury suggests that the sulfur dioxide and chlorine can be passed to a catalytic reactor with additional sulfur dioxide and oxygen to form sulfur trioxide and chlorine but cautions that a chlorine resistant catalyst should be used.
A. W. Hixson, et al in their U.S. Pat. No. 2,441,550 suggest the reaction of salt and sulfur trioxide and also the step of converting sulfur dioxide to sulfur trioxide, cooling the sulfur trioxide, absorbing it in oleum and thereafter heating the oleum to release the sulfur trioxide for the reaction with the salt. The article "Chlorine and Salt Cake from Salt and Sulfur" by A. W. Hixson and A. H. Tenney in the December, 1941 issue of Industrial and Engineering Chemistry reports that reactions of sulfur trioxide and sodium chloride took 96 hours at room temperature and 23 hours at 60.degree. C. They also suggest that the intermediate product, sodium chloro-sulfonate, may be valuable as a sulfonating agent.
A. W. Hixson, et al in their U.S. Pat. No. 2,553,767 suggest that the separation of sulfur dioxide and chlorine is to be accomplished by reacting the sulfur dioxide with a material which is inert to chlorine and with which sulfur dioxide forms an addition compound, from which the sulfur dioxide may be evolved or liberated by elevation of temperature. The patent suggests that anhydrous aluminium chloride could be used.
In a report entitled "The Separation of Sulfur Dioxide and Chlorine" dated Dec. 20, 1944 for the Office of Production Research and Development of the War Production Board with the University of Wisconsin by Olaf A. Hougen, the difficulties of separating sulfur dioxide from chlorine and many of the various processes that might be tried for such separation are discussed.
From these publications it is readily apparent that the proposed process for the production of chlorine by the reaction of sulfur trioxide and sodium chloride is fraught with many problems. Further applicants are not aware of any commercial venture which has used this reaction for the production of chlorine, even through knowledge of the reaction has been available for many years.